This invention relates to magnetic resonance imaging apparatus.
The invention especially relates to apparatus for producing images which suppress fluid, particularly apparatus using inversion recovery sequences.
One such sequence is Fluid-Attenuated Inversion Recovery (FLAIR).
FLAIR provides a highly sensitive means of detecting lesions of the brain, particularly in the periventicular region. However CSF flow can lead to poor CSF suppression, particularly in the brain stem, the foremana of Munroe and around the spinal cord. This problem can be avoided by the use of a non-selective inversion pulse, which inverts all the CSF, so that flow ceases to make any difference. A disadvantage in implementing this concept is that, the sequential excitation of the subsequent slices results in an inversion time that varies from slice to slice. This leads to variable contrast from one slice to the next with incomplete suppression of CSF in many slices.
Recently a method has been presented (Norris, D. G., ESMRMB Proc. 264, 1999) of reducing RF dose for MDEFT imaging at high field by using a single non-selective inversion pulse followed by a rapid stream of field echo slice excitations per TR. This method was subject to variable contrast due to the variable TI per slice, and this was dealt with by cycling the slice excitation order and associating each TI with a region of K-space rather than a particular slice.
In accordance with the present invention, slice order cycling is applied to the FLAIR sequence.
This allows uniform contrast to be attained combined the desirable flow insensitive properties of non-selective inversion pulse. The cycling provides a mechanism which can ensure that data for vhich CSF is at its null point is placed at the centre of k-space in each slice. The sequence has interesting properties because of the very different state of magnetisation recovery of brain/cord and CSF.